continuous integration: Add support for coveralls via github actions

[SimonKagstrom]

Can probably be based on https://github.com/coverallsapp/github-action.

The main issue seems to be to pass the GITHUB_REPOSITORY etc environment variables to the run command (i.e., kcov). Then the service_name should be set to github and the GITHUB_RUN_ID variable should be used as build_num in payload.